(a) Field of the Invention
Broadly speaking, this invention relates to radiation detection. More particularly, in a preferred embodiment, this invention relates to an improved radiation detection tube having a useful life which is at least one order of magnitude greater than that therefore obtained.
(b) Discussion of the Prior Art
Radiation detectors find widespread application in both civil and military environments. By far the most common detector is the Geiger-Mueller tube which is less expensive and more rugged than the scintillation counter.
Unfortunately, the Geiger-Mueller tube has been found to have a limited life, which is believed to be caused by cathode degradation in the region of the insulator-cathode seal. For example, prior to the instant invention, commercial G-M tubes, especially small-sized units, had useful lives of from 10.sup.9 to 10.sup.10 counts when operated at moderate loadings and moderate over-voltages. The end of useful tube life occurs when the counter no longer yields an accurate measure of gamma-radiation. The end of useful life could result, for example, from the end of the tube's plateau voltage falling below the operating point. This reduction in plateau length results from increased secondary electron emission from the tube's cathode and manifests itself as multiple tube counts.
Investigation has revealed that the undesirable secondary emission is caused by damage to the cathode's passivation layer. More particularly, within the G-M tube, the abrupt termination of the glass frit seal and the metal electrodes distorts the applied electric field creating field intensities of such magnitude that the aforementioned damage to the passivation layer can occur. The above observations have been confirmed by the use of a scanning electron microscope which clearly revealed the existence of an abrupt insulator-metal interface in the cathode seal region of several commercial G-M tubes. Of course, in such tubes the secondary emission would increase with rising voltage to the point where multiple tube counts would cause the G-M tube to go into continuous discharge.